What you were expecting to happen, and what actually happened?
Enemy team had Moloch, I had Goblin King. Goblin King had 30 Life, Moloch’s ability deals 8 true damage. I expected to survive. I didn’t.
Moloch had reduced Goblin King to 14 life. goblin King was in the last slot of my team, everyone else was still alive. Moloch used his ability to deal 8 true damage to Goblin King. Instead he got the normal attack animation and dealt 14 true damage (same as his attack value), bypassing Goblin King’s 12 armour and killing him.

What are the steps to make it happen again?
No idea, sorry.

Do you have any screenshots or video you want to share with us so we can see the problem? Attach them to your post!
Nope.

Was the goblin king charged? Every two mana boosts the true damage by 1. So if he had 12 mana the +6 would have dealt the 14 killing the king.

As to the mob doing the normal damage animation, I have that a lot, but very unpredictably. Happens on my casts, and on enemy casts. I even have had my front line troop “attack” (animation only) all my other troops when an AoE is cast from the enemy.